Pneumonectomy (removal of a whole lung) is a life-saving procedure in patients who cannot otherwise be cured; but it carries considerable morbidity and mortality. While pneumonectomy results in rapid growth of the remaining lung, the stimuli and molecular mechanisms of this compensatory lung growth (CLG) remain unknown. Understanding these mechanisms will answer important questions concerning CLG and organ regeneration. Thus the long-term objectives of this proposal are to understand the molecular mediators that trigger and regulate CLG. Nitric oxide (NO) is an important regulator of CLG, and is known to mediate the angiogenic and mitogenic properties of key growth factors in endothelial cells and type II alveolar cells. Thus this proposal will test the overall hypothesis that NO is a key mediator of CLG through the modulation of angiogenesis and type II cell proliferation. To test this hypothesis, knockout mice deficient in endothelial NO synthase (eNOS), inducible NOS (iNOS), and both eNOS/iNOS will be used to study NO-mediated mechanisms of post-pneumonectomy CLG. Specific Aim 1 will test the hypothesis that NO is required for CLG by modulating angiogenesis and type II cell proliferation and differentiation. Angiogenesis and type II cell proliferation and differentiation will be measured in the NOS-deficient mice. Specific Aim 2 will test the hypothesis that NO is essential to CLG by modulating expression of specific proliferation factors in the lung. Expression of cytokines, NOS, growth factors and their receptors will be measured. Augmentation of CLG will be tested by administration of exogenous growth factors, and use of inhaled NO will determine the times during which NO is critical to CLG. Specific Aim 3 will test the hypothesis that a mechanism of NO- regulated CLG is via the cGMP signaling pathway and the MAPK cascade. Activation of cGMP-dependent kinase I and MAPK kinase will be determined, and CLG will be measured in mice after the inhibition of cGMP-dependent kinase I using a selective pharmacologic inhibitor. Identification of the molecular mediators in regenerative lung growth will not only advance the field of lung biology and transplantation, but will permit the advancement of future therapies for lung injury and end-stage lung disease through the stimulation and control of lung growth and regeneration.